The polyphenylene ether resins are a well-known class of thermoplastic materials which are commercially available. Polyphenylene ether resins are flammable and require the use of additives to achieve a commercially acceptable degree of flame retardancy.
Polyvinyl aromatic resins, such as polystyrene or high impact polystyrene (HIPS), are another well-known class of thermoplastic materials which are commercially available. As with polyphenylene ether resins, the polystyrene resins, including HIPS, are flammable and require the use of additives to achieve a commercially acceptable degree of flame retardancy.
If a thermoplastic test bar held in a test stand burns vigorously enough for a sufficient time, the bar will gradually soften (absent an unexpected cross-linking reaction) and will begin to exhibit sagging. As the bar becomes softer and less viscous, a point will be reached at which the bottom portion of the bar will separate physically from the remainder of the bar retained in the test stand. This phenomenon of physical separation is known as "dripping" and poses danger in certain applications because the dripping resin can ignite adjacent materials.
In the prior art, blends of polyphenylene ether and HIPS have been effectively rendered self-extinguishing in the UL Subject 94 vertical burn test through the addition of aromatic phosphate esters. However, these flame-retarded self-extinguishing compositions sometimes fail in the UL Subject 94 5 V flammability test due to dripping and/or flaming resin which physically separates from the test bar.
Thus, in the prior art, there have been problems in the preparation of flame retardant compositions comprising a polyphenylene ether resin, alone, or in combination with a polystyrene resin (including HIPS) which do not form flaming drips when ignited with a direct flame.
Haaf et al., U.S. Pat. No. 4,107,232, found that it is possible to achieve a flame retardant, non-dripping polyphenylene ether composition by the addition of a flame retardant compound and from 0.1 to 0.25 parts by weight per 100 parts of the composition of polytetrafluoroethylene. The compositions are said to optionally include a vinyl aromatic resin which may be a homopolymer (e.g., styrene), modified homopolymer (e.g., rubber-modified, high impact polystyrene) and copolymers (e.g., styrene butadiene copolymers, ABS copolymers and styrene-butadiene block copolymers).
In Haaf et al., U.S. Pat. No. 4,332,714, there are disclosed drip-retardant, plasticized thermoplastic compositions comprising, in admixture, a polyphenylene ether resin, a plasticizer in an amount at least sufficient to provide a plasticized composition after molding, and a microfibrillar poly(tetrafluoroethylene) resin in an amount at least sufficient to render the thermoplastic composition non-dripping when molten.
Thus, in the prior art, polyphenylene ether resins have been rendered non-dripping through the use of polytetrafluoroethylene resin.
However, poly(tetrafluoroethylene) is extremely costly and is difficult to blend or melt compound into polyphenylene ether resin-based compositions as a "neat" additive (i.e. without the aid of a compounding aid) using currently available processing equipment. For example, "nesting" (a phenomenon wherein fibrils of PTFE are formed in the production apparatus) often occurs during low shear blending. Excessive die swell and surging result in "dropped strands" which are encountered when the poly(tetrafluoroethylene) is not homogeneously dispersed into such polyphenylene ether/polystyrene compositions during compounding.
In addition, while the halogen concentration in poly(tetrafluoroethylene)-modified flame retardant polyphenylene ether/polystyrene resins is very low, in view of the increase in regulatory concern over the toxicity and corrosivity of the combustion products of halogenated compounds in various end-use applications, for example, in telecommunication, computer and/or other business equipment, a need still exists to develop a cost effective, non-halogen-containing drip-inhibitor as an alternative to poly(tetrafluoroethylene) in polyphenylene ether-based resin compositions.
Gowan, U.S. Pat. No. 3,361,851 discloses the addition of low density polyethylene resin or polypropylene resin in concentrations of up to about 10% by weight in polyphenylene ether resin, and this has been shown to result in improvements in the impact strength and stress cracking resistance of the polyphenylene ether resins. It also has been shown that such low density polyethylene or polypropylene resin could be used in blends of polyphenylene ether resin/polystyrene resin for a broad range of processing and property improvements. However, at polyolefin concentrations greater than about 2% by weight, lamination was often seen in injection-molded parts.
Lee, Jr., U.S. Pat. No. 4,166,055, discloses that a styrene-butadiene copolymer (e.g., the Kraton.RTM. G block copolymers) could serve as an effective compatibilizer for polyphenylene ether/polyolefin blends.
To date, it is believed that the use of high molecular weight polyethylene resin as an additive to polyphenylene ether-based resin compositions has not been reported. In fact, there is no mention of use of these high molecular weight polyethylene resins as an additive of any sort in current product literature, or in two recent review articles. Caughlan et al., "UHMWPE" Encyclopedia Polymer Science & Engineering, 2nd Edition, Vol. 6, pp. 490-492 (1986); and Miller, "UHMWPE" Modern Plastics Encyclopedia, Vol. 66, No. 11, pp. 75-76 (1989).